The present invention relates to a dry end of a paper making machine. This is an improvement upon and a modification of the dry end shown in an application filed on the same date as this one based on German Application G 91 00 762.3, filed Jan. 24, 1991.
The dry end consists of a plurality of dryer groups. Each comprises at least one and typically a plurality of dryer cylinders. The dryer cylinders in each group alternate with belt reversal rolls. A respective support belt for a web passes through each dryer group and meanders alternately to partially wrap each dryer cylinder and the following reversal roll. The invention is particularly concerned with the arrangement of reversal rolls at the transfer regions between adjacent dryer groups where the web is transferred from the support belt of a preceding dryer group to the support belt of a succeeding group. Features of such a dry end are known from Federal Republic of Germany Utility Model 90 01 209.7, which corresponds to U.S. application Ser. No. 07/467,788, filed Jan. 19, 1990. The dry end dries a fiber web, for instance, a web of paper, particularly in a paper making machine, which is designed for a very high operating speed. The highest operating speed can be about 1500 m/min, or even more. In order to achieve this, the support belts for the web are preferably porous dryer wires and the reversing rolls are preferably suction guide rolls which hold the web against the outside of the support belt in each dryer group as the web passes around the reversing rolls, so long as the support belt and the web travel together from one dryer cylinder to the next one in sequence.
In the above mentioned Utility Model, the web is transferred from a first dryer group comprised of a first group of dryer cylinders to a second dryer group comprised of a second group of dryer cylinders. A first suction guide roll of the second dryer group serves as a removal roll. The first support belt of the first dryer group travels around a final suction guide roll and then travels tangentially to the circumference of the removal roll, around which the second support belt of the second dryer group also travels. In front or upstream of the removal roll in the direction of travel, the first and second support belts form a so called angle of convergence, which may amount to between about 2.degree. and 30.degree.. This configuration is more favorable than another known arrangements also using a removal roll in which the two support belts travel a distance parallel to each other in front of the removal roll, where the web is located between the two support belts. With this parallel guidance, there is the danger that the web, which is still moist, may be subjected to injurious stressing, particularly if the two belts travel at a certain speed differential.
According to the above mentioned Utility Model, the support belt of the second dryer group comes into contact over a small portion of the circumference of the removal roll with the support belt of the first dryer group. This means that the support belt of the first dryer group wraps around the circumference of the removal roll over a small angular sector. In addition, it is proposed that this angle of wrap be variable during the operation of the machine. In this way, it is possible to transfer the web with a high degree of safety, i.e. without a substantial danger of it tearing, from the first support belt of the one dryer group to the second support belt of the next dryer group. This is true even for very high operating speeds, because impermissible stressing of the web can be avoided. If necessary, the angle of wrap may even be zero.
Finally, the above mentioned Utility Model takes into account that at times one dryer group must be shut down while the others continue to operate. For this event, provision is made temporarily to establish a distance at the suction pickup roll of the second dryer group between the support belts of the first and second dryer groups.
Although the dry end of a paper making machine described above has proven satisfactory in practice, further improvements are desirable. There is a disadvantage of the known dry end. The paper web tears during normal operation of the drying section. This can never be completely avoided. The torn web is not present between the two support belts of two adjacent dryer groups. As a result, the two support belts travel temporarily in direct contact with each other over the suction pickup roll, which is usually the first roll of the following dryer group. Although the joint path of travel of the two support belts here is only short and although this operating condition generally lasts only for a brief time, the temporary mutual contact of the two support belts, which, as mentioned above, are developed as dryer wires, appears to cause a certain amount of wear on the outsides of the support belts.